This invention relates to a composite transistor circuit, and more particularly to a transistor circuit intended to equivalently elevate the current amplification factor .beta..
The conventional differential amplifier consisting of PNP transistors is generally composed of a circuit shown in FIG. 1 or 2. Referring to FIG. 1, PNP transistors Q1, Q2 jointly constitute a differential transistor circuit constructed by connecting a common emitter to a reference current source 11. The bases of said PNP transistors Q1, Q2 are respectively applied as a noninverted input terminal 13 and inverted input terminal 14. Connected to the collector side of the transistors Q1, Q2 is a current mirror circuit comprising NPN transistors Q3, Q4. The collector of noninverted input transistor Q1 is connected to the base of NPN transistor Q5 constituting an emitter-grounded amplifier circuit. An output is drawn off from the collector of said transistor Q5. Numeral 12 is a reference current source, and numeral 15 is a bias terminal.
If, in the above-mentioned circuit, the transistors Q1, Q2 have low current amplification factors .beta., and a high resistance element RH has to be connected to the base of the transistor Q1, then the base current of the transistor Q1 causes a voltage drop across both ends of said high resistance element RH to such an extent as can not be overlooked. Therefore, care should be taken in designing the captioned composite transistor circuit. Now let it be assumed that the collector current I.sub.C of the transistor Q1 is 10 (.mu.A), and the current amplification factor .beta. of said transistor Q1 is 10. Then the base current of the transistor Q1 stands at I.sub.C /.beta.=1 (.mu.A). Further, if it is assumed that the aforementioned high resistance element RH has a resistance of 100 k.OMEGA., then the voltage appearing across both ends of said high resistance element RH rises as high as 100(k.OMEGA.).times.1(.mu.A)=100 (mV). Since such a prominent voltage drop appears across the output terminal in the form of offset voltage, great care should be taken in designing the captioned composite transistor circuit.
In this connection, a description may now be made with reference to FIG. 2 of a circuit which has already been proposed to equivalently elevate the current amplification factor .beta. of the noninverted side transistor in order to reduce the base current appearing at the noninverted input terminal 13 of the differential amplifier so as to minimize voltage drop. With this proposed circuit, a PNP transistor Q6 is Darlington connected to a transistor Q1. Further, a PNP transistor Q7 is also Darlington connected to a transistor Q2. The above-mentioned Darlington connection is intended to attain a balance between the inverted and noninverted sides of the aforesaid differential amplifier.
Discussion may now be made of a voltage drop occurring across both ends of the high resistance element RH provided on the noninverted side of the aforementioned differential amplifier. Now let it be assumed that I.sub.CQ1 shows the collector current of a transistor Q1; I.sub.BQ1 denotes the base current of said transistor Q1; I.sub.EQ6 represents the emitter current of a transistor Q6; I.sub.CQ6 sets forth the collector current of said transistor Q6; and I.sub.BQ6 shows the base current of said transistor Q6. Further, let it be supposed that the current amplification factor .beta. of both transistors Q1, Q6 are represented by .beta..sub.P alike. In this case I.sub.BQ1 is expressed as: ##EQU1##
The following formula is generally accepted: EQU I.sub.E .apprxeq.I.sub.C
Therefore, I.sub.EQ6 .apprxeq.I.sub.CQ1 ##EQU2## Therefore, As seen from the above formula (1), the base current of the transistor Q6 corresponds to 1/(.beta..sub.P).sup.2 of the collector current of the transistor Q1. If, therefore, the transistors Q1, Q6 are assumed to jointly constitute one composite transistor, then the amplifying factor .beta. of said composite transistor may be equivalently expressed as (.beta..sub.P).sup.2, providing that the amplifying factor .beta. is increased over that of the circuit of FIG. 1.
Further referring to FIG. 2 representing another prior art, a description may now be made of a voltage drop occurring across both ends of the resistor RH connected to the noninverted input terminal. As in the circuit of FIG. 1, the following assumption ia made: EQU I.sub.CQ1 =10(.mu.A)
The amplification factor .beta. of the transistors Q1, Q6=10. Then, the base current I.sub.B of the composite transistors Q1, Q6 is expressed as: ##EQU3##
Therefore, RH=100 (k.OMEGA.) connected to the noninverted input terminal of the differential amplifier is expressed as: ##EQU4## The above formula is also applicable to a voltage drop occurring across both ends of a high resistance element (not shown) provided on the inverted side of the differential amplifier. Assuming, therefore, that the Darlington connection is provided as shown in FIG. 2, it is possible to reduce the base currents supplied to the noninverted and inverted input terminals to such an extent as can be overlooked (that is, to equivalently increase the amplification factor .beta. of the transistor). Namely, the prior art circuit of FIG. 2 is more improved than that of FIG. 1
If the amplification factor .beta. is fixed in the prior art circuit of FIG. 1, the base current of the transistor Q1 can be decreased by reducing the reference current I.sub.O1. In such a case, however, it is impossible to obtain sufficient drive current to actuate the transistor Q5. The prior art circuit of FIG. 1 has the drawback that since the base current component of the transistor Q1 increases to such a large extent as can not be overlooked, said circuit of FIG. 1 can not be applied as a high input impedance circuit. Now referring again to the prior art circuit of FIG. 2, if an input transistor is Darlington connected to another transistor, then said composite transistor circuit can allow for the passage of a large reference current I.sub.O1 required to produce a sufficient drive current to actuate the transistor Q5. In this case, it is also possible to reduce the base current of the transistor Q6. However, the prior art transistor circuit of FIG. 2, where an equivalent base-emitter voltage is twice as large as that in FIG. 1, is not adapted to be used as a circuit requiring a minimum (-0.9 V) drive source voltage.
The merits and demerits of the prior arts of FIGS. 1 and 2 may be summarized as in the following table.
______________________________________ Merits and demerits FIG. 1 FIG. 2 ______________________________________ Drive capacity obtained Insufficient Sufficient by a small reference current I.sub.O1 Effect exerted by a base Large Small current running through a noninverted input terminal Operation under the Good Not good ordinary minimum source voltage (V.sub.CCmin = 0.9 V) ______________________________________